Gas assisted laser cutting apparatus

ABSTRACT

Apparatus for subjecting a workpiece to the action of a laser beam comprises a nozzle and optical means for directing the laser beam at the workpiece through the nozzle, together with means for directing a gas stream at the workpiece through the same nozzle.

United States Patent 1191 Sullivan et a1.

[ July 31, 1973 GAS ASSISTED LASER CUTTING APPARATUS [75] Inventors:Arthur Basil Joseph Sullivan,

Letchworth; Peter Thomas Houldcroit, Royston, both of England GB [73]Assignee: National Research Development Corporation, London, England[22] Filed: Jan. 19, 1971 [21] Appl. No.2 107,675

Related US. Application Data [62] Division of Ser. No. 712,782, March13, 1968, Pat.

[52] US. Cl. 219/121 L, 250/495, 331/945 A 511 1111. C1 B23k 27/00 581Field ofSearch, 219/69, 121 EB, 121 L, 219/130, 137, 70, 74, 343;331/945; 250/495 [56] References Cited UNITED STATES PATENTS 7/1969Hafner 219/121 LM Great Britain 55,346/67 2/1965 Radtke et al. 214/121EB 3,156,810 11/1964. Samuelson 214/121 EB 3,383,491 1 5/1968 Muncheryan219/121 L 3,242,314 3/1966 Eckles 219/343 3,388,314 6/1968 Gould 219/121L 3,364,087 1/1968 Solomon et 219/121 EM 3,369,101 2/1968 Di Curcio219/121 L 3,204,081 4/1965 Iceland 219/125 PL 3,417,222 12/1968 Cannonet al. 219/121 EB 3,360,398 12/1967 Garibotti 219/121 LM 3,226,52712/1965 Harding 219/121 L OTHER PUBLICATIONS Research on Lasers andPlasma Jets BF Scott, International Journal of Machine Tool Research,Vol. 5, pp. 11-24 (1965).

Primary Examiner-J. V. Truhe Assistant Examiner-George A. MontanyeAttorney1(emon, Palmer & Estabrook 57 ABSTRACT Apparatus for subjectinga workpiece to the action of a laser beam comprises a nozzle and opticalmeans for directing the laser beam at the workpiece through the nozzle,together with means for directing a gas stream at the workpiece throughthe same nozzle.

14 Claims, 7 Drawing Figures PATENTEUJULWQB v 3.749.878

7 sum u (1F 4 JIMMY;

, 1 GAS ASSISTED LASER CUTTING APPARATUS rough machining. The reason forthis inaccuracy is that the heating cannot be sufficiently concentrated,especially where a gas stream, such as oxygen,'used to sweep away themolten metal is of a kind having an exothermic reaction with thepreheated area of the metal. This exothermic reaction is self-sustainingwhen the temperature of the workpiece reaches a predetermined value andconsequently the width of the cut varies with the width of the stream ofoxygen which is directed at the preheated area. In addition to theinaccuracy of the cut, the existing thermal methods have the furtherdisadvantage that heating over a considerable area of the workpiecetends to damage it, for example by distorting it or affecting itsproperties or structure.-

As a heat source, the laser beam permits an energy concentration of anexceedingly high order and for some years the laser has been known as apiercing instrurnent where fine holes are required. Moreover, it can befocused to a small spot with considerable accuracy and a laser which candeliverenergy to the workpiece at a rate sufficient to boil the metal inits path can achieve a cutting process. Consequently, the laser has beenconsidered as a cutting instrument for metals, to make very narrow cutssuch as would not be possible with conventional thermal cutting methods.

We have found that a particularly efiective form of apparatus fordirecting a gas stream at the point on a workpiece at which a laser beamis concentrated includes, in addition to optical means for directing thelaser beam on to the workpiece, a nozzle'through which the laser beamemerges from the apparatus. in the direction of the workpiece, andmeans'for directing a gas stream through the same nozzle towards theworkpiece substantially coaxially with the laser beam.

In the said application Ser. No. 712,782, we have described andclaimedamethod of cutting completely through a workpiece in which alaser beam is concen trated on a workpiece and the beam and workpieceare moved relatively to one another and in which a jet of a gascapableof taking part in an exothermic reaction is directed on to themoving region of the workpiece at which the laser beam is concentratedto provoke an exothermic reaction, this being continued, until theworkpiece is cut through, the reactive gas jet'then sweeping away theproducts of combustion through the.

mined largely by the laser beam and not by the gas jet, which controlledthe width of the cut in the earlier processes of thermal cutting andgouging which utilised an exothermic reaction. In the said copendingapplication, we have put forward some possible reasons for thissurprising behaviour. Briefly, we believe that the strip which is beingcut is raised very rapidly to a temperature which is considerably higherthan that associated with conventional cutting processes and that as aconsequence the proportion of the heat input which is consumed as latentheat in melting and vaporising, and

thus is removed from the body of the metal by the gas -s tream,is muchmore significant than in the case of the earlier processes of thermalcutting. There is, therefore, little spreading of the zone which is at atemperature high enough to take part in the exothermic reaction. Theabsence of spreading due to self-sustaining exothermic reaction is alsoevident in the direction of the cut and this provides an additionaladvantage in that the cutting process can be stopped very rapidlybyswitching the laser ofi. Consequently, the method lends itselfparticularly well to machine-controlled thermal cutting,;in which amachine responsive to instructions programmed on a record medium may bearranged to switch on and off the laserbeam and to start, stop andchange the direction of relative movement between the nozzle and theworkpiece.

For a cutting or gouging operation, the apparatus embodying theinvention is provided with driving means for effecting relative movementbetween the nozzle and theworkpiece in the required direction of theoperation. In addition, the apparatus advantageously in- 7 cludes aguide system for the laser beam to permit modcut. Apparatus embodyingthe present inventioncan be and fineness of the cut are substantiallyunimpaired by;

the addition of the gas stream, in spite of the fact that thecross-sectional areaof this stream at the workpiece may differ by anorder of magnitude from that of the laser beam. It seems that the widthof the cut is deterification ofthe path of the beam between the laserand the nozzle.

Although wecan cut stainless steel and also Nimonic and other refractorymetals without a fluxing agent using an exothermicallymeacting gas,there are some materials for which a fluxing agent is useful. Forexample, ceramic material, brick, tile, concrete, stone, rock and glassdo not react exothermically with oxygen and to cut these materials witha laser beam -we introduce.

a metal or acompound which permits an exothermic reaction with the gasand creates a fluxing action with the material to be cut at the pointwhere the laser beam strikes this material. The gas jet then sweeps thefused material or the low melting point mixtures away from the heatedarea. As an example, the fluxing agent may be fed in powdered form in agas jet to the heated area and may be iron or may be formed of halidesof the alkali metals. In another form the fluxing agent is coated on tothe surface of the material to be cut, for example by painting orspraying.

With most of the materials mentioned above, iron may be used'as 'afluxing agent in conjunction with anoxygen jet. For example, iron powdermay be fed into the laser system in an oxygen jet. The iron powderoxidises rapidly and provides super-heated iron oxide which fluxes andcuts the ceramic or other material. Al-

ternatively, the iron may be fed in as wire or strip at the point wherethe gas jet and the laser beam meet. A further possibility is to feed avolatile or vaporised halide (for example a halide of iron) into anoxygen jet directed at the portion of the workpiece heated by the laserbeam. The combination of iron as a fluxing agent with an oxygen jet canalso be used for aluminium, which also forms refractory oxides whenheated.

The nozzle through which the beam and gas jet emerge, although extremelycompact, is not subjected to substantial heating and thus the intensepotential heating of the laser beam is safely combined with an activegas such as oxygen, for example. This is contrary to experience in thecoarse" forms of thermal cutting employing a reactive gas, in whichsubstantial heating of the nozzle takes place. In fact, the plasma torchwhich, until the use of the laser for thermal cutting, achieved thehighest form of convergence and concentration of heat, it was notpossible to use a gas producing an exothermic reaction because the hotparts of the torch would be rapidly oxidised and eroded, and as a.consequence only an inert gas could be used, the molten metal beingswept away by the inert gas.

For gouging, an oblique arrangement of the nozzle relative to theworkpiece is necessary and more than one laser can be used. For thepurposes of this specification, the term gouging" is intended to includemachining operations such as parting a bar in a lathe, tuming andplaning.

In order that the invention may be better understood some examples ofapparatus for carrying the inventionv into effect will now be describedwith reference to the accompanying drawings, in which:

FIG. 1 shows diagrammatically the arrangement of a first form ofapparatus;

FIGS. 20, 2b and 20 show diagrammatically the cut-v ting and gouging ofmetals;

FIG. 3 shows a form of nozzle for imparting a circular movement to thelaser beam at the surface of the workpiece;

FIG. 4 shows the laser cutting head mounted on a transverseplatform;'and

FIG. 5 shows in section an adjustablenozzle.

In FIG. 1, the laser is of the carbon dioxidenitrogen-helium type andhas a length of 10 metres and a bore of 30mm. There are five sections tothe tube, only two being shown in the diagram, and each section has itsown electrodes 12 and its own electrical supply 14. The power supplyprovides k.V. for striking a 9 k.V. at 45m.A. when the laser is running.The laser body 16 has a fully reflecting concave mirror 18 ofgold-surfaced stainless steel at one end and a plane semi-reflectinggermanium disc 20 at the other end. The concave mirror has a focallength slightly in excess of 10 metres. The gas in which the dischargeis to be created flows into the laser body through the inlet 24 and outfrom the laser body through the outlet 22.

It should be understood that the design and operation of the laser formno part of the present invention and it will be sufficient to state thatwhen the laser is in op-' eration a substantially parallel beam ofcoherent light emerges from the laser through the disc 20. In thepresent case, the laser was operated from an AC supply'of 50 Hz andconsequently the outputwas pulsed at 100 pulses per second. I 1

The emergent beam 26 is reflected by an aluminised mirror 28 through asafety shutter 30 to a focusing lens 32. From this lens, the beam passesthrough a window 34 into an oxygen chamber 36. Oxygen enters thischamber through an inlet 38 and emerges in the direction of theworkpiece through a nozzle 40. The axis of e parabolic mirror suitablypositioned off the optical axis. A focal position l/l6 inch beyond thenozzle has been found satisfactory when using an oxygen jet of diameter0.10 inch; this arrangement gives a clean and stable cut.

As indicated by the arrow 44, the workpiece is moved horizontally topermit the laser, assisted by the oxygen stream, to make a cut. As thecut proceeds, the molten or vaporised material is ejected through thecut zone by the oxygen jet whichalso serves to provide an exothermicreaction, as described above.

We have achieved cuts of from'0.0l5 inch to 0.025

inch width in mild steel," high carbon tool'st'eel and stainless steelof 0.10 inch thickness at speeds up to 40 inch per minute, provided thata minimum energy density is achieved. It appears that the addition ofthe oxygen jet does not greatly affect the width of the cut, althoughthe addition of the oxygen jet permits cutting of metal several timesthicker than what can be achieved with the unaided laser. The width ofcut is governed primarily by the diameter and energy of the laser beam,but also. by the speed of movement and the thermal diffusivity of theworkpiece. The narrow cut gives a precisionnot previously obtained withoxyacetylene cutting. Increased laser power enables-a higher cuttingspeed but with an AC-operated laser, the speed is limited by the pulsednature of the supply. DC operation removes this difficulty. i i 1 Thesmoothness and stability of cutting depends upon exceeding a minimumspeed which varies according to the laser power and the material whichis being cut. Below this speed, (or below a threshold power) the laserloses control and the melting begins tp propagate beyond the areadirectly heated by the laser. This results in melting large holes and inintermittent cutting operation. As the speed increases, the laser againassumes control and gives a fast clean cut of small width. The thresholdlaser power is allied to the minimum speed, since if the power, isinsufficient the threshold speed of cutting cannot be achieved. Forcutting mild steel of 0.10 inch thickness at a speed of 40 ins/min, wefound that it was desirable for the laser power to exceed 300 watts. Forthickness of 0.030 inch, a power of 250 watts gave a cutting speed of 55ins/min, and a power of watts gave a speed of 17%ins/min. For athickness of 0.060 inch, the cutting speed with a power of 250 watts wasreduced to 17%ins/min. It may in some cases be desirable to provideextra power for starting and this can be achieved by generating aninitial laser pulse of an energy greater than its continuous rating.

In FIG. 1, the laser beamand gas jet strike the workpieceperpendicularly to its surface. However, the nozzle through which thebeam and jet emergecan bearrangedat an angle to the workpiecesurface,as-shown in FIG. 20,-. FIG. 2b shows a gouging operation in whichandlaser and we believe that this is because the molten material is notblown straight through the workpiece, as in the case of a cut, butinstead gives up some of its heat energy on its way up the sides of thegroove.

and the pair of nozzles a workpiece portion of V- shaped cross-sectionis removed.

For cutting thicker workpieces, it may be desirable to increase thewidth of the cut, as the width of the cut at the base might otherwise beinsufficient to allow, the

passage of enough gas. To increase the effective beam diameter at theworkpiece surface, the laser beam can be slightly defocused, thatis tosay brought to'a focus above the work surface, provided that the laserhas sufficient power. As an alternative to defocusing the laser beam,the beam can be given a cyclically repetitive movement transverse to thedirection of the cut. A nozzle of the kind shown in FIG. 3 can be used.In this figure, the beam 26, on emerging from the focusing lens 32impinges on a mirror 58 mounted axiallyon a re-' volving ring 60. Thebeam reflected from the mirror 58 is again reflected by a mirror 62mounted at the edge of the ring 60. The ring 60 is driven by a pinion 64so that the mirror 62 rotates bodily around the axis of the nozzle. Thearrangement is such that the focused laser beam at the surface of theworkpiece 42 describes a succession of small circles on the latter,these circles being symmetrical about the line of the cut resulting fromthe relative movement of the workpiece and noz- 5 zle. With a suitablerelationship between the circular movement and the linear movement, theoverlap of successive circles can be such that all elements in the trackof the circle are scanned by the laser beam. However, we have 'foundthat when a laser beam scans in a circle in the way described, thereaction tends to spread into'the small area of metal surrounded by thecircle. Consequently, it is possible to cut through for the whole widthof the circle without a complete overlap, the laser beam diameter andthe oxygen supply being so chosen that the spread of the reaction isjust enough to remove the centres of the circles formed by the movementof the laser beam. The gas jet flows through the nozzle towards theworkpiece, as shown in FIG. 3. If the line of cut turns, the circularmovement still imparts to the laser beam the same motion transverse tothe direction of the cut.

To avoid inserting a driving shaft through the wall of the laser tube,the ring 60 can be driven by an induction motor if desired. Otherpossibilities are to use a rotating prism to provide the circularmovement of the beam or to arrange a transparent plate in the pa-th of'the laser beam at an angle to the beam, sothat the emergent. beam isparallel to but offset with respect to the incident beam. The plate orblock is now rotated about the axis of the incident beam to cause theemergent beam to sweep out a circle. An elliptical be'am movement couldbe produced by two elements arranged in series in the path of the beamand at 90 to one another, the

chamber through a series of holes 68 in theinner wall 7 of the diffusingring.

A rack 70 and pinion 72 are provided to permit adjustment of the cuttinghead.

FIG. 4 shows the cutting head 74 mounted on an x-y traverse platform topermit the head to be positioned at any point over the surface of aworkpiece 42 without movement of the laser 10. The cutting head 74 ismounted on a block 76 which also carries the oxygen supply tube 78a. Theblock 76 is mounted on a lead screw 78, the rotation of which providesthe y traversing movement, the lead screw78 being, rotated by a slidingworm 80 on a splined shaft 82driven by a motor 84. The x traversingmovement is obtained by the rotation of a lead screw 86 by a motor 88,theshaft 78 being supported on a block 90 engaged on the lead screw 86.The parallel beam 26 is reflected by the prisms 92 and 94 beforereaching the nozzle 74.

In the apparatus shown in FIG. 5, the height of the nozzle above theworkpiece is adjustable by means of y a telescopic connection 139. Thelaser beam 26 is reflected by a 45 reflector l40into the telescopicarm,which terminates in a nozzle of the kind having a difius ing ring 66 ofthe kind shown in FIG. 3;

The nozzle and aligned guide tube shown in FIG. 5

can 'be used in the apparatus of FIG. 4.

The guide tube of FIG. 5 also readily permits the containment of a gasfor protecting the optical reflecting surfaces. With a carbon dioxidelaser, for example, generating radiation at a wavelength of 10 microns,the

' less precautions are takemA dry warmgas, such as ni windows and lenseshave to be of special material. G er-.

manium can be used but is expensive and the cheaper potassium bromidesuffers from the disadvantage that it is hydroscopic and thereforeliable to deteriorate untrogen, in theguidetube system preventsdeterioration of the potassium bromide surfaces.

Among'refractory metals cut by the combination of a laser and anexothermically reacting gas are tantalum, a tantalum alloy containing 10percent tungsten and aniobium alloy containing 10 percent tungsten. Wehave also cut Nilo K, an iron-nickel alloy (Nilo is a Registered TradeMark).

We claim:

1. Apparatus for subjecting a workpiece to the action of a coherentlight beam, comprising: a laser generating a coherent light beam;opticalmeans for directing the laser beam through the apparatus towardsa workpiece;

a nozzle through which the laser beam emerges from' the apparatus in thedirection of the workpiece; and means for directing at the workpiecethrough the said nozzle, a gas which is exothermically reactive with theworkpiece and under sufficient pressure to sweep away the resultingreaction products.

"2. Apparatus for subjecting a workpiece to the action of a coherentlight beam, comprising: a laser generating a coherent light beam;tubular means surrounding the laserbeam; optical means supported by thetubular meansfor focussing the laser beam on to a workpiece;

a nozzle at the end of thesaidtubularmeans, the laser beam emergingthrough the said nozzle in-the direction angle of each to itsincidentbeam beingaltered in an In FIG. 3,the oxygen entering the nozzlethrough the inlet 38 passes into a diHusing ring66 and enters the of theworkpiece; and means for directing at theworkpiece through the saidnozzle, a gas'stream which is exothermically reactive with the workpieceand under sufiicient pressure to sweep away resulting reaction products.

3. Apparatus in accordance with claim 2, further comprising: drivingmeans for effecting relative movement between the said nozzle and aworkpiece in the required direction of a path of action of the coherentlight beam along the workpiece; a light-directing device supported bythe tubular means and arranged in the path of the laser beam; and meansfor moving the lightdirecting device in such a manner that at thesurface of the workpiece, the laser beam executes a cyclicallyrepetitive movement transverse to the direction of the said relativemovement.

4. Apparatus in accordance with claim 3, in which the said lightdirecting device is given a movement of rotation to cause the laser beamat the surface of the workpiece to execute a circular movementsymmetrical about the center line of the said relative movement.

5. Apparatus in accordance with claim 2, further including guiding meansfor the laser beam, the guiding means, comprising: a first lead screwsupporting the said nozzle for movement in response to rotation of thelead screw; first internally threaded means mounted on the said leadscrew, the said nozzle being supported on the internally threaded means;a reflector supported by the said internally threaded means; a secondlead screw; a second internally threaded means mounted on the secondlead screw and carrying the first lead screw; a further reflectorsupported by the second internally threaded means; the laser and thereflectors being so arranged that the beam is directed through the exitof the nozzle, the position of which is governed by the rotation of thesaid lead screws.

6. Apparatus for subjecting a workpiece to the action of coherent light,comprising:

laser means for generating first and second coherent light beams;

first and second optical means for directing the first and second beams,respectively, through the apparatus towards a workpiece;

mounting means for the said laser means and optical means whereby saidfirst and second beams are directed on to the said workpiece at an angleto one another so that they converge to a point in the neighbourhood ofthe surface of the workpiece to make parallel cuts which converge to apoint below the surface of the workpiece;

and first and second jet forming nozzles mounted on said mounting meansso that the first and second laser beams emerge respectively through thesaid nozzles in the direction of the workpiece;

and means for supplying gas to said nozzles at a pressure such that onemerging from the nozzles the gas sweeps away molten or vaporizedmaterial.

7. Apparatus for subjecting a workpiece to the action of a coherentlight beam, comprising: i i

a laser generating a coherent light beam;

optical means for connecting the laser beam onto a workpiece;

a jetforming nozzle having an outlet through which the laser beamemerges from the apparatus in the direction of the workpiece;

a housing surrounding at least a portion of the laser beam and extendingback from said nozzle towards said laser, said housing having a gasinlet spaced from said nozzle outlet and communicating through saidhousing with the interior of said nozzle; and

means for supplying gas to said inlet at a pressure such that onemerging from the nozzle the gas sweeps away molten or vaporizedmaterial.

8. Apparatus in accordance with claim 7, further comprising:

driving means for effecting relative movement between the said nozzleand a workpiece in the required direction of a path of action of thecoherent light beam along the workpiece; I

a light-directing device supported by the said housing and arranged inthe path of the laser beam;

and means for moving the light-directing device in such a manner that atthe surface of the workpiece the laser beam executes a cyclicallyrepetitive movement transverse to the direction of the said relativemovement.

9. Apparatus in accordance with claim 8, in which said light-directingdevice is given a movement of rotation to cause the laser beam at thesurface of the workpiece to execute a circular movement symmetricalabout the centre line of the said relative movement.

10. Apparatus in accordance with claim 7, further including guidingmeans for the laser beam, the guding means comprising:

a first lead screw supporting the said nozzle for movement in responseto rotation of the lead screw; first internally threaded means mountedon the said lead screw, the said nozzle being supported on theinternally threaded means; i

a reflector supported by the said internally threaded means;

a second lead screw;

a second internally threaded means mounted on the second lead screw andcarrying the first lead screw;

a further reflector supported by the second internally threaded means,the laser and the reflectors being so arranged that the beam is directedthrough the exit of the nozzle, the position of which is governed by therotation of the said lead screws.

11. Apparatus as defined by claim 7 including a transparent shieldsupported by said housing on that side of said gas inlet remote fromsaid nozzle outlet to prevent flow of gas from said inlet toward saidlaser.

12. Apparatus as defined in claim 7 in which said nozzle includes adetachable tip portion.

13. Apparatus for subjecting a workpiece to the' action of a coherentlight beam, comprising:

a laser generating a coherent light beam;

a head assembly remote from said laser and terminating in a nozzlehaving an outlet through which the laser beam emerges from the apparatusin the direction of the workpiece;

a lens mounted in said head assembly for focusing said coherent lightbeam on to the workpiece; a gas inlet in said head assemblycommunicatingwith said nozzle outlet, said nozzle outlet forming the gas introducedby said gas inlet into a gas jetdischarging from said nozzle outlet onto said workpiece and coaxial with said laser beam:

and means for adjusting the position of the head assembly normal to theworkpiece independently of the laser, whereby the laser beam can befocused said gas inlet into a gas jet discharging from said nozzleoutlet on to said workpiece;

driving means for achieving relative movement between said nozzle on theone hand and said workpiece and said laser on the other hand;

and guiding means controlling the direction of such relative movementand'permitting said nozzle to follow a nonlinear path relative to saidworkpiece.

k I! I l UNITED STATES iATENT OFFICE f CERTIFICATEO-F CORRECTION. PatentNo. :3; 749,878 o v Dated. 'j' j| y" 3 19 73 In ve cofls) AR *IAI-I URB. SULLIVA and PET Efi THOMAS HoULDCROF'f It is certifieo tjzhat "erorappeerefin the ebove-identified patent v V and that seid LettersPatent are hereby corrected as vshown below:

.IN' THE CLAIMS:

Column" 7, line 57, change "connecting" to i 5""-'COncent.

Signed end sealed this 11th day of March-1975.

, (SEAL) Attest v I e C. MARSHALL DANN RUTH C. MASON, I V v vCommissioner of Patents Attesting Officer and Trademarks UNITED STATESPATENT QFFICE CERTIFICATE OF COREQTKN Patent No. 3 749,878 Dated- July197 Inventor(s) ARTHUR B. J. SULLIVAN and PETER THOMAS HOULDCROFT It iscertified that error appearsin the above-identified patent and that saidLetters Patent are hereby corrected as shown below:

. IN THE CLAIMS:

Column 7, line 57, change "connecting" to -conce'ntrating--.

Signed and sealed this 11th day of March 1975.

(SEAL) Attest 2 C. MARSHALL DANN RUTH C. MASON Commissioner of PatentsAttesting Officer and Traclemautks

1. Apparatus for subjecting a workpiece to the action of a coherentlight beam, comprising: a laser generating a coherent light beam;optical means for directing the laser beam through the apparatus towardsa workpiece; a nozzle through which the laser beam emerges from theapparatus in the direction of the workpiece; and means for directing atthe workpiece through the said nozzle, a gas which is exothermicallyreactive with the workpiece and under sufficient pressure to sweep awaythe resulting reaction products.
 2. Apparatus for subjecting a workpieceto the action of a coherent light beam, comprising: a laser generating acoherent light beam; tubular means surrounding the laser beam; opticalmeans supported by the tubular means for focussing the laser beam on toa workpiece; a nozzle at the end of the said tubular means, the laserbeam emerging through the said nozzle in the direction of the workpiece;and means for directing at the workpiece through the said nozzle, a gasstream which is exothermically reactive with the workpiece and undersufficient pressure to sweep away resulting reaction products. 3.Apparatus in accordance with claim 2, further comprising: driving meansfor effecting relative movement between the said nozzle and a workpiecein the required direction of a path of action of the coherent light beamalong the workpiece; a light-directing device supported by the tubularmeans and arranged in the path of the laser beam; and means for movingthe light-directing device in such a manner that at the surface of theworkpiece, the laser beam executes a cyclically repetitive movementtransverse to the direction of the said relative movement.
 4. Apparatusin accordance with claim 3, in which the said light directing device isgiven a movement of rotation to cause the laser beam at the surface ofthe workpiece to execute a circular movement symmetrical about thecenter line of the said relative movement.
 5. Apparatus in accordancewith claim 2, further including guiding means for the laser beam, theguiding means, comprising: a first lead screw supporting the said nozzlefor movement in response to rotation of the lead screw; first internallythreaded means mounted on the said lead screw, the said nozzle beingsupported on the internally threaded means; a reflector supported by thesaid internally threaded means; a second lead screw; a second internallythreaded means mounted on the second lead screw and carrying the firstlead screw; a further reflector supported by the second internallythreaded means; the laser and the reflectors being so arranged that thebeam is directed through the exit of the nozzle, the position of whichis governed by the rotation of the said lead screws.
 6. Apparatus forsubjecting a workpiece to the action of coherent light, comprising:laser means for generating first and second coherent light beams; firstand second optical means for directing the first and second beams,respectively, through the apparatus towards a workpiece; mounting meansfor the said laser means and optical means whereby said first and secondbeams are directed on to the said workpiece at an angle to one anotherso that they converge to a point in the neighbourhood of the surface ofthe workpiece to make parallel cuts which converge to a point below thesurface of the workpiece; and first and second jet forming nozzlesmounted on said mounting means so that the first and second laser beamsemerge respectively through the said nozzles in the direction of theworkpiece; and means for supplying gas to said nozzles at a pressuresuch that on emerging from the nozzles the gas sweeps away molten orvaporized material.
 7. Apparatus for subjecting a workpiece to theaction of a coherent light beam, comprising: a laser generating acoherent light beam; optical means for connecting the laser beam onto aworkpiece; a jetforming nozzle having an outlet through which the laserbeam emerges from the apparatus in the direction of the workpiece; ahousing surrounding at least a portion of the laser beam and extendingback from said nozzle towards said laser, said housing having a gasinlet spaced from said nozzle outlet and communicating through saidhousing with the interior of said nozzle; and means for supplying gas tosaid inlet at a pressure such that on emerging from the nozzle the gassweeps away molten or vaporized material.
 8. Apparatus in accordancewith claim 7, further comprising: driving means for effecting relativemovement between the said nozzle and a workpiece in the requireddirection of a path of action of the coherent light beam along theworkpiece; a light-directing device supported by the said housing andarranged in the path of the laser beam; and means for moving thelight-directing device in such a manner that at the surface of theworkpiece the laser beam executes a cyclically repetitive movementtransverse to the direction of the said relative movement.
 9. Apparatusin accordance with claim 8, in which said light-directing device isgiven a movement of rotation to cause the laser beam at the surface ofthe workpiece to execute a circular movement symmetrical about thecentre line of the said relative movement.
 10. Apparatus in accordancewith claim 7, further including guiding means for the laser beam, theguding means comprising: a first lead screw supporting the said nozzlefor movement in response to rotation of the lead screw; first internallythreaded means mounted on the said lead screw, the said nozzle beingsupported on the internally threaded means; a reflector supported by thesaid internally threaded means; a second lead screw; a second internallythreaded means mounted on the second lead screw and carrying the firstlead screw; a further reflector supported by the second internallythreaded means, the laser and the reflectors being so arranged that thebeam is directed through the exit of the nozzle, the position of whichis governed by the rotation of the said lead screws.
 11. Apparatus asdefined by claim 7 including a transparent shield supported by saidhousing on that side of said gas inlet remote from said nozzle outlet toprevent flow of gas from said inlet toward said laser.
 12. Apparatus asdefined in claim 7 in which said nozzle includes a detachable tipportion.
 13. Apparatus for subjecting a workpiece to the action of acoherent light beam, comprising: a laser generating a Coherent lightbeam; a head assembly remote from said laser and terminating in a nozzlehaving an outlet through which the laser beam emerges from the apparatusin the direction of the workpiece; a lens mounted in said head assemblyfor focusing said coherent light beam on to the workpiece; a gas inletin said head assembly communicating with said nozzle outlet, said nozzleoutlet forming the gas introduced by said gas inlet into a gas jetdischarging from said nozzle outlet on to said workpiece and coaxialwith said laser beam: and means for adjusting the position of the headassembly normal to the workpiece independently of the laser, whereby thelaser beam can be focused on to the workpiece without moving the laser.14. Apparatus for subjecting a workpiece to the action of a coherentlight beam, comprising: a laser generating a coherent light beam; anenclosure surrounding at least a portion of the length of the laserbeam; optical means supported by the enclosure for focusing the laserbeam on to a workpiece; said enclosure terminating in a nozzle in thepath of said laser beam and having an outlet through which the laserbeam emerges from the apparatus in the direction of the workpiece and agas inlet spaced from and communicating with said nozzle outlet, saidnozzle outlet forming the gas intorduced by said gas inlet into a gasjet discharging from said nozzle outlet on to said workpiece; drivingmeans for achieving relative movement between said nozzle on the onehand and said workpiece and said laser on the other hand; and guidingmeans controlling the direction of such relative movement and permittingsaid nozzle to follow a nonlinear path relative to said workpiece.